1. Field of the Invention
The present invention relates to a phosphor preparation method. Specifically, the present invention relates to a preparation method of a rare earth-activated, alkaline earth metal fluorohalide based accelerated-phosphorescent phosphor for use in radiation image recording and reproducing methods.
2. Description of the Related Art
Conventionally, there has been known a bivalent europium-activated barium fluorohalide phosphor (BaFX: Eu.sup.2+ ; wherein X is halogen other than fluorine) which emits light (instantaneous emission of light) in ranges from a near-ultraviolet region to a blue beam region by excitation with radiation such as X-ray, electron beam, or ultraviolet beam. The phosphor is used as that for a radiosensitization screen employed in radiography or the like.
Further, there has recently been found that, when the above-described phosphor is exposed to radiation such as X-ray, electron beam, or ultraviolet beam, and thereafter, excited with electromagnetic wave (excitation light) in ranges from a visible region to a red beam region, the phosphor emits light (accelerated-phosphorescent emission) in ranges from a near-ultraviolet region to a blue beam region (which will be hereinafter referred to as accelerated-phosphorescent characteristics). Thus, the above-described phosphor has been noted especially as a phosphor useful for radiation image conversion panels. Such panels are typically radiation image recording and reproducing methods, which employ accelerated-phosphorescent phosphor.
In particular, a phosphor containing iodine as a part of halogen X has a high accelerated-phosphorescent luminance. As the amount of iodine therein increases, a peak of an accelerated-phosphorescent excitation spectrum shifts to a range of longer wavelength. Accordingly, there has been proposed a method in which the phosphor is used in combination with a laser which emits light whose wavelength is in a red beam region (for example, He--Ne laser), or a semiconductor laser which emits light whose wavelength is in a red beam region or an infrared region in accordance with a content amount of iodine.
The above-described radiation image recording and reproducing method is constructed in such a manner that an accelerated-phosphorescent phosphor of a radiation image conversion panel is allowed to absorb radiation energy transmitted through a subject or emitted from an object to be examined. Thereafter, the accelerated-phosphorescent phosphor is excited by an electromagnetic wave in accordance with a time series to thereby allow the radiation energy stored in the accelerated-phosphorescent phosphor to emerge as fluorescence and an electric signal is obtained by photoelectrically reading the fluorescence. Finally, a visual image is produced based on the obtained electric signal, such as on a recording material such as a photosensitive film, and/or on a display device such as a CRT.
In carrying out radiation image conversion methods, the radiation image conversion panel itself generally deteriorates very little even with radiation or electromagnetic wave being irradiated thereto. Therefore, the radiation image conversion panel can be used repeatedly over a long period of time. Usually, the radiation energy stored in the panel is read by scanning a laser beam across the panel. However, the radiation energy cannot be completely emitted or erased from the panel, only by using a scanning laser beam. Accordingly, in order that radiation energy remaining in the panel be emitted, there has been proposed, as is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 56-11392 and the like, a method in which remaining radiation energy be erased by causing light of accelerated-phosphorescent luminescence in an excitation wavelength region to be irradiated onto the panel after the reading.
However, there has been found that when the radiation energy stored in a panel using an accelerated-phosphorescent phosphor which contains iodine is erased in a short time (for example, several seconds to several minutes in a daylight fluorescent lamp), which is applied to a case in which an ordinary accelerated-phosphorescent phosphor is used, remaining energy cannot be sufficiently erased, and further, a portion of the remaining energy is recovered (an after image emerges) with the passage of time after the erasing. When the panel is used repeatedly, the above-described erasability and after-image characteristic adversely affects the quality of an image. Further, when the time for erasing is increased, the total time required for reading and erasing increases to an extent, to cause deterioration in processing ability of the device and generation of heat in an eraser.
The above-described phosphor is generally prepared by the following method. First, a mixture of phosphor materials is prepared by a dry process in which the phosphor materials are homogeneously mixed in a dry state, or a wet process in which the phosphor materials are homogeneously mixed together in a slurry state and then dried. Next, the mixture of the phosphor materials is fired ordinarily at a temperature near a melting point of a host crystal (Ba, FX, and the like) in a near reducing atmosphere or in a neutral atmosphere in several hours. The obtained fired product may be further fired if desired. The firing allows growth of the host crystal of the phosphor, and at the same time, diffuses activator elements in the host crystal. Further, an F.sup.+ -center which serves as a central source of an accelerated phosphorescence is also generated. Accordingly, the firing is an important process which exerts an influence upon light emission characteristics of the phosphor. After the firing, the obtained phosphor is subjected to washing, classification, and the like, if necessary.
Further, JP-A Nos. 7-233369 and 10-195431 each disclose a method for preparing a rare earth-activated, alkaline earth metal fluorohalide based accelerated-phosphorescent phosphor (which may be hereinafter referred to simply as "phosphor") having a tetradecahedral structure in which grain shape and grain aspect ratio are controlled. In a radiation image conversion panel having an accelerated-phosphorescent phosphor layer in which a rare earth-activated, alkaline earth metal fluorohalide based accelerated-phosphorescent phosphor having a tetradecahedral structure (which may be hereinafter referred to simply as "tetradecahedron-structured phosphor") is provided, the tetradecahedron-structured phosphor is configured with a low directionality in the accelerated-phosphorescent phosphor layer, and therefore, the undesirable transverse extension of the excitation light and the accelerated-phosphorescent emission is lessened and the sharpness of a resultant radiographic reproduction image improves. The emission characteristics, particularly, sharpness, of the phosphor obtained by the preparation methods disclosed in the above-described publications are extremely high. However, there has been demand for further improvement in sensitivity and erasability of the phosphors when the phosphors are used in radiation image recording and reproducing methods.
A method for improvement in erasability of an accelerated-phosphorescent phosphor is disclosed in, for example, JP-A No. 8-231952. This method comprises the steps of: firing a mixture of materials to obtain an intermediate product. The intermediate product is thereafter annealed at a temperature lower than the firing temperature in a flow of a slightly oxidized atmospheric gas. However, this method has problems in that operations such as decision of an intermediate product, control of respective temperatures during the firing and annealing, determination of an annealing time, and determination of conditions that oxidized gas flows, are complicated. Further, it is not clear what conditions contribute to erasability.
As described above, under the existing circumstances, factors for determining the above-described erasability and after-image characteristic of accelerated-phosphorescent phosphor have not been made clear. For this reason, the best conditions for preparation which are provided to improve the erasability and after-image characteristic cannot be set, and a problem arises in stability of the preparation.